As is shown in FIG. 12, there exists an extruder 10, which extrudes a material supplied from a material inlet port 4 to a die part 5 by rotating a screw 1 inserted in a barrel 2 that corresponds to a housing and has a heater, and the screw 1 is rotated by a driving part 3. The extruder 10 transfers while heating and kneading the material in the barrel 2 (for example, with reference to a cited document 1: published specification of Japanese Patent Laid-Open No. 5-50424). As the extruder 10 in this configuration, there is a type having one screw (i.e., one screw shaft) 1 and another type having two screws 1 (i.e., two screw shafts). The twin-screw type extruder with two screws 1 meshed with each other is divided into one in which each screw 1 rotates in the same direction as shown in FIGS. 13 and 15, and one in which each screw rotates in a different direction as shown in FIG. 14. Also with respect to a shape of the screw (screw shaft) 1, there are a trapezoidal screw shape as indicated in FIGS. 13 and 14, and a waveform screw shape as indicated in FIG. 15 other than the trapezoidal screw shape.
Furthermore, there are extruders having three or more screws. These extruders are constructed in such forms that many screws are arranged circumferentially as indicated in FIG. 16, with two sets each consisting of two mutually engaged screws separately arranged while the screws are not engaged with each other between the sets. Each screw in the same set rotates in the same direction while the screws are made to rotate in different directions between different sets. Conventionally, no extruder is present in which three or more of the aforementioned waveform screws are arranged horizontally and meshed with each other to rotate in the same direction.
For the twin-screw extruder with two screws engaged as described above, an increase in the quantity of the material to be extruded (that is, an increase in the treatment volume) is required. To cope with this, in FIG. 17, an outer (large) diameter D of the screw shaft 1 is increased and a root (small) diameter 17 of the screw 1 is decreased. In other words, a depth 16 of the thread grooves of the screw shaft 1 is made larger regardless of the rotation direction and the shape of the screw in the conventional art. Here, “to increase the screw diameter” is taken for the case that the screw diameter exceeds approximately 90 mm.
However, to increase the screw outer (large) diameter results in an elongated facility length, and also an increase in mechanical loss and an increase in driving current of the screw shaft due to a large rotation weight of the screw shaft or the like. Moreover, since the barrel shape is enlarged as well, it takes time for the heater to raise temperatures. When a large screw shaft is designed, normally, a sectional shape of the large screw is made geometrically similar to a sectional shape of a small screw, and a value of L/D which is a ratio of a screw outer circumference moving distance when the screw rotates once and a groove depth of the screw is made constant. Under this condition, a peripheral velocity of the large screw is faster as compared with the small screw, whereby a self-heating value because of kneading of the material present in thread grooves of the screw is increased and the material is degraded. Furthermore, the above speeding up of the peripheral velocity necessitates considerations with respect to abrasion of the screw and the barrel.
Meanwhile, to increase the groove depth of the screw 1 makes a flight angle 19 of the screw denoted in FIG. 17 smaller. This reduction of the flight angle 19 decreases an abrasion resistance of the screw shaft 1, and at the same time increases a leak amount of the material from the screw shaft 1. Since a shaft diameter 18 of the screw shaft 1 is reduced when the groove depth 16 of the screw 1 is increased, it brings about a problem in terms of a strength of the screw 1.
Taking the above problem into consideration, to increase the screw diameter (that is, to increase the groove depth for increasing the treatment volume) is not necessarily a good solution. In general, when an extruder with a large diameter screw is to be designed, experiments are carried out with the use of an experimental extruder with a small diameter screw, and a mix proportion of materials to be treated, a screw shape and the like at the actual machine with the large diameter screw are determined on the basis of the acquired empirical data. However, it is reality that the actual machine does not always bear the same result as designed because of a difference in the screw diameters between the experimental machine and the actual machine, and the like.
The present invention is devised to solve the above-described problems, and has for its object to provide an extruder capable of obtaining a treatment volume equal to that of the conventional art without increasing a screw diameter.